Method for applying a protective coating to the components of a printed circuit board

ABSTRACT

Method for applying a protective coating to the components on a printed circuit board, said coating being comprised of a polymerizable material such as a resin, characterized in that it consists in preparing the material of which the coating is composed from an initial mixture of at least two ingredients (A, B) and adding to this initial mixture at least one third ingredient (C) in a variable quantity in order to vary the viscosity of the initial mixture according to the type of component to be protected during the coating of the printed circuit board components.

BACKGROUND OF THE INVENTION

The present invention relates to a method for applying a protectivecoating to the components of a printed circuit board, said coating beingcomprised of a polymerizable material such as a resin.

Protection of components of printed circuit boards used in a harshenvironment (temperature, moisture, impacts, vibrations, etc.) byapplying to them a coating comprised of a polymer resin is known. Suchan operation is often called "varnishing" or "tropicalization" ofboards.

In general, a protective coating is made of a single layer of resinapplied by sprinkling, or by spraying, or by dipping the board in aliquid resin bath.

However, the protection provided by this resin layer often proves to beinadequate. The complexity of printed circuit boards, due to thepresence of active and/or passive components of highly variable size andfragility such as connectors, the various housings of electronic orintegrated-circuit components, etc. requires protective coatings ofdifferent types depending on the type of component in question. Toaccomplish this, different molds and different types of resins have tobe made for selectively applying an appropriate coating to each type ofboard component. Specifically, connectors require a coating based on aviscous resin for impact strength in particular, while components of SMD(surface mounted device) housings require protection against moisturefor example by means of a highly fluid resin for small housings or asemi-viscous resin for larger housings. As a result, protection of thevarious components on a single printed circuit board requires productionof a resin-holding mold for each component type so that the resin isapplied selectively without flowing beyond the component to beprotected.

This being the case, protection of the components on a printed circuitboard requires prior fabrication of several molds and a coatingoperation for each mold used. Implementation is lengthy and complex,resulting in very high manufacturing costs.

SUMMARY OF THE INVENTION

The goal of the invention is to remedy these drawbacks and, for thispurpose, the invention provides a method for applying a protectivecoating to the components on a printed circuit board, characterized inthat it consists in preparing the material of which the coating iscomposed from an initial mixture of at least two ingredients and addingto this initial mixture at least one third ingredient in a variablequantity in order to vary the viscosity of the initial mixture accordingto the type of component to be protected during the coating of theprinted circuit board components.

According to another characteristic of the method according to theinvention, the two ingredients of the initial mixture are an isocyanateand a polyol, with these two ingredients reacting with one another toform a resin.

According to another characteristic of the invention, the thirdingredient for varying the viscosity of the initial mixture is athixotropic agent or a catalyst.

In general, the method according to the invention consists in producingthe initial mixture in a mixing unit equipped with an ejection nozzleand, according to another go characteristic of the method, the thirdingredient is advantageously added to the initial mixture in an outputchamber of the unit that communicates with the ejection nozzle, theprotective coating then being ejected from the nozzle and being appliedselectively to the various components, depending on the viscosityrequired for each type of board component, by displacing the nozzlerelative to the board or vice versa.

According to one important advantage of the invention, it is possible tocoat the components on a printed circuit board with a protective coatingmade from one and the same initial mixture whose viscosity can be variedaccording to the type of component to be protected, using a singleapparatus which, in a single operation or pass, is capable of coatingall the components to be protected that protrude on at least one side ofthe printed circuit board.

According to another advantage of the invention, the apparatus requiredfor implementing the method can be designed totally independently fromthe individual features of each printed circuit board, in particular thedimensions of the board and the number of components it supports.

According to yet another advantage of the invention, the apparatusrequired for implementing the method is considerably less expensive thanthat used in the prior art.

Other advantages, characteristics, and details of the invention willemerge from the explanatory description hereinbelow with reference tothe attached drawing that schematically illustrates the principle of themethod according to the invention.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows an apparatus for applying a protective coating to thecomponents of a printed circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Protection of the components on a printed circuit board envisagedaccording to the invention consists in applying, at least to each of thecomponents, a coating formed from an initial mixture of at least twoingredients to obtain a polymerizable material such as a resin.

This initial mixture is formed from a first ingredient A such as anisocyanate and a second ingredient such as a polyol. These twoingredients A and B react with each other to form a resin with a certainviscosity. As an example, the isocyanate is of the toluene isocyanatetype or the diphenylmethane-4,4'diisocyanate type and the polyol used isof the polyether-polyol or polyester-1-polyol type.

At least a third ingredient C is added to this initial mixture, saidingredient being intended to vary the viscosity of the initial mixtureand being a thixotropic agent or a catalyst. As an example, thethixotropic used is a 2,4-bis(p-aminobenzyl)aniline and the catalystused is an organometallic compound such as a dibutyltin dilaurate

Specifically, with reference to the attached drawing, the twoingredients A and B of the initial mixture and the third ingredient Cintended to vary the viscosity of this initial mixture are stored inthree reservoirs R1, R2, and R3, respectively.

The two reservoirs R1 and R2 are connected by a tube 1 to a mixing unit5 where they are mixed. Mixing unit 5 advantageously has an outputchamber 6 that is in communication with an ejection nozzle 8. The thirdreservoir R3 is connected by a tube 1 to output chamber 6 of mixing unit5 to vary the viscosity of the initial mixture just before its ejectionthrough nozzle 8. Mixing unit 5 is for example displaceable in threedirections so that it can move relative to printed circuit board CEwhich remains fixed. The means for displacing mixing unit 5 areillustrated schematically and numbered 9.

A processing and control unit comprised of a programmable robot 10 isconnected by electrical conductors 12 to displacing means 9 to transmitcontrol commands and thus displace unit 5 in three directions relativeto board CE. In addition, this robot 10 is connected by electricalconductors 14 to a valve V3 located at the outlet of reservoir R3 tocontrol the quantity of third ingredient C which is introduced intomixing unit 5 to vary the viscosity of initial mixture A+B as it leavesnozzle 8, depending on the type of component to be protected during agiven operation protecting all the components on board CE.

Robot 10 can also be used to control the quantities of the twoingredients A and B introduced into mixing unit 5. For this purpose,electrical conductors 15 and 16 connect the robot to two valves V1 andV2 mounted at the outlets of the two reservoirs R1 and R2, respectively.

Specifically, the three components P1, P2, and P3 to be protected on aprinted circuit board CE are a connector, a medium-sized SMD housing,and a small SMD housing. Robot 10 is programmed to protect these threecomponents in a preset order which for example is a function of theviscosity of the coating specific to each component type and the desiredprotection. The order chosen is for example P1, P2, then P3 in order toapply a coating of the viscous, semi-viscous, then fluid type from thesame initial mixture whose viscosity is varied as ejection nozzle 8moves.

In general, depending on the viscosity desired, the third ingredient Cis added to initial mixture A+B in proportions by volume such that A+Bis approximately 99.9 to 90% and C is approximately 0.1 to 10%. As anexample, protection of connector P1 against impacts or vibrationsrequires a coating with a viscosity greater than 70,000 mpa.s, moistureprotection of the medium-sized SMD housing or component P2 will requirea coating with a viscosity of 5000 to 70,000 mPa.s, and moistureprotection of the small SMD housing or component P3 will require acoating with a viscosity of less than 5000 mPa.s. These values areprovided for a one-kilogram mixture at a temperature of 22° C.

Robot 10 is able automatically to program variations in viscosity of theinitial mixture in output chamber 6 of mixing unit 5 before ejecting itthrough nozzle 8 to apply a protective coating to each type of componentduring a given operation.

Thus, depending on the type of component to be protected on printedcircuit board CE, the viscosity of the coating can be varied as nozzle 6moves over board CE. advantageously, when the coating is applied in theliquid form to a component of the board, this component can first besurrounded by a dam of viscous coating to prevent the coating fromflowing beyond the component.

Once all the components on one side of printed circuit board CE arecoated with a protective coating, board CE is placed in an oven topolymerize the coating. Thereafter, any components located on the otherside of the printed circuit board are protected after said board hasbeen turned over. In general, the printed circuit board isadvantageously protected on each of its two sides so that it is entirelycoated with a protective coating. To protect each end edge of the boardor field of the board, a protective dam can be applied to the peripheryof one side of the board with a viscosity such that the dam can overflowor slump to cover the field of the board.

We claim:
 1. A method for applying a protective coating to varyingcomponent types on a printed circuit board with a single apparatus in asingle pass, said coating being comprised of a polymerizable material,the method comprising the steps of:preparing an initial coating materialmixture of at least two ingredients in a mixing unit of the singleapparatus equipped with an ejection nozzle; adding to the initialmixture at least one third ingredient in a variable quantity in order tovary a viscosity of the initial mixture according to a first type ofcomponent on the printed circuit board to be protected to form a firstprotective coating, the at least one third ingredient being added to theinitial mixture in an output chamber of the mixing unit, the outputchamber communicating with the nozzle; coating the first type ofcomponent on the printed circuit board with the first protective coatingby ejecting the first protective coating from the nozzle during a firstpass of the single apparatus; adding to the initial mixture said atleast one third ingredient in a variable quantity in order to vary theviscosity of the initial mixture according to a second type of componentlocated on the printed circuit board to be protected to form a secondprotective coating differing in viscosity from the first protectivecoating, said at least one third ingredient being added in the outputchamber; and coating the second type of component on the printed circuitboard with the second protective coating by ejecting the secondprotective coating from the nozzle during the first pass of the singleapparatus across the printed circuit board.
 2. The method according toclaim 1, characterized in that it comprises using an isocyanate as afirst ingredient of the initial mixture.
 3. The method according toclaim 2, characterized in that the isocyanate is one of a toluenediisocyanate and a diphenylmethane-4,4-diisocyanate.
 4. The methodaccording to claim 1, characterized in that it comprises using a polyolas a second ingredient of the initial mixture.
 5. The method accordingto claim 4, characterized in that the polyol is one of apolyether-polyol and a polyester-polyol.
 6. The method according toclaim 1, characterized in that it comprises using a thixotropic agent asthe third ingredient to vary the viscosity of the initial mixture. 7.The method according to claim 6, characterized in that the thixotropicagent is a 2,4-bis(p-aminobenzyl)aniline.
 8. The method according toclaim 1, characterized in that it comprises using a catalyst as thethird ingredient to vary the viscosity of the resulting mixture.
 9. Themethod according to claim 8, characterized in that the catalyst is anorganometallic compound such as a butyltin dilaurate.
 10. The methodaccording to claim 1, wherein the first and second protective coatingsare ejected from the nozzle and applied selectively to the first andsecond components, respectively, by relative displacement between thenozzle and the board during the first pass.